1. Field of the Invention
This invention relates to umbrellas, and more particularly, to a collapsible umbrella (also called folding umbrella) with a reliable and smooth self-opening mechanism.
2. Description of Related Art
A collapsible umbrella is typically composed of a stretchable multi-segment shaft, an upper fixed hub mounted on the top end of the shaft, a running hub slidably mounted on the shaft which can move along the shaft in either directions to open or collapse the umbrella, a collapsible frame of radiating ribs which are linked to the upper fixed hub and the running hub, and a fabric held on the collapsible frame to serve as a cover against rain or shine. Moreover, a finishing cap is mounted on the top of the upper fixed hub. In the umbrella industry, however, there are two standards for the mounting of the finishing cap on the upper fixed hub: one is the outer-threaded type (USA standard) which is formed with a threaded bolt that can be screwed into a threaded hole formed in the upper fixed hub; and the other is the inner-threaded type (Japanese standard) which is formed with a threaded hole that allows a threaded bolt formed on the upper fixed hub to be screwed thereinto. Apparently, these two standards are incompatible. Therefore, the umbrella manufacturers have to provide two different types of finishing caps and upper fixed hubs to meet the different standards in different countries, which considerably increase the difficulty in manufacture as well as in inventory management, and thus the manufacturing cost is high.
Further, as shown in FIG. 15, to help the self-opening operation of the collapsed umbrella to be more smooth, an assembly of an elastic piece and a coil spring is customarily mounted on each pair of an inner branch rib and a middle branch rib of the collapsible frame. The elastic piece is a bent wire having one end (the bent portion) linked to the inner branch rib and the other end (the two tips) inserted in the coil spring. Further, the coil spring is mounted in a groove formed from a U-shaped section of the middle branch rib, which is secured firmly in position by forcibly narrowing the portion of the U-shaped section of the middle branch rib where the coil spring is mounted. This method of firmly securing the coil spring in position, however, requires high precision to perform in order to reliably secure the coil spring in position without fear of being self dismounted due to poor machining or after a long period of use. This requirement can, however, considerably increase the manufacturing cost of the umbrella. Moreover, after a long period of repetitive uses, the securing means for the coil spring can be deformed such that it can no long secure the coil spring firmly in position, causing the coil spring to be likely to fall off the middle branch rib. Should this be the case, the self-opening mechanism of the collapsible umbrella will fail, and the user has to open the umbrella manually.
Still another drawback of the conventional collapsible umbrellas is that the upper fixed hub is customarily formed with a polygonal shape, which can somewhat cause the collapsing and opening of the collapsible frame to be unsmooth. The use of the collapsible umbrella is thus inconvenient.